Field of the Invention
The present invention relates to a unidirectional condenser microphone and its manufacturing method with which a polar pattern of the microphone can selectively be set from a cardioid pattern to a hyper cardioid pattern.
Description of the Related Art
For using a microphone, it is desired that a directivity of the microphone is appropriately selected in accordance with sound-collecting condition.
For example, for microphones belonging to a class of unidirectional microphones, in the case of collecting a sound of an instrument such as a drum, a microphone with a hyper cardioid characteristic is preferably used. And in the case of using a microphone for amplifying vocal sound or the like, a microphone with a cardioid characteristic is preferably used.
The unidirectional condenser microphone is generally configured to capture a bi-directional component into a backside of a diaphragm through an acoustic resistor provided at a rear acoustic terminal side.
Thus, in the unidirectional condenser microphone, it is technically important to set a stable acoustic impedance. Setting of the acoustic impedance affects performance of the unidirectional condenser microphone, thus, various proposals have been conventionally made.
For example, the applicant of the invention has proposed a condenser microphone with a stable acoustic resistance by adopting a device for controlling passage of a sound wave through a surface boundary between an insulating base for supporting a fixed electrode and an acoustic resistance material provided at the front side of the insulating base. This is disclosed in JP 2010-183249 A.
In addition, the applicant of the invention has proposed a configuration in which the fixed electrode is bonded to an electrode extraction part that supports the fixed electrode in JP patent application No. 2013-136102.
The unidirectional condenser microphone with a stable acoustic impedance can also be obtained by using this configuration and sandwiching the acoustic resistance material between the fixed electrode and the electrode extraction part.
In the above-mentioned unidirectional condenser microphones, the acoustic impedance is set by the acoustic resistance material in order to obtain desired directional characteristics.
Therefore, fine control of the acoustic resistance material is required for obtaining the cardioid or hyper cardioid directional characteristics as described in the beginning. This fine control of the acoustic resistance material is heavily dependent on experience and intuition of a person in charge of manufacturing.
Further, there is a problem that the acoustic impedance is not stable enough because the acoustic resistance material shows significant change over time.
For these reasons, there was a problem of variations of characteristics caused by difference among people in charge of manufacturing or deterioration of the acoustic resistance material in the conventional techniques. It was difficult to set the acoustic impedance of the unidirectional microphone stably, and there was a problem in productivity and stability of the characteristics in the conventional techniques.